The present invention relates to semiconductor fabrication and, more particularly, to a method for making a stainless steel reinforced belt pad for linear chemical mechanical planarization (“CMP”).
In the fabrication of semiconductor devices, CMP is used to planarize globally the surface of an entire semiconductor wafer. CMP is often used to planarize dielectric layers as well as metallization layers. As is well known to those skilled in the art, in a CMP operation a wafer is rotated under pressure against a polishing pad in the presence of a slurry. In a linear CMP operation, the polishing pad is typically a belt pad. One known belt pad includes a molded layer of polyurethane disposed on a stainless steel band, which provides reinforcement for the layer of polyurethane.
As the size of integrated circuits continues to decrease, the need for a CMP operation to produce uniformly smooth planarized surfaces increases. In a linear CMP operation, any variation in the thickness of the pad, i.e., the molded layer of polyurethane, can adversely affect the smoothness of the surface being planarized. One drawback of the molding process typically used to form stainless steel reinforced belt pads is that variations in the size and roundness of the stainless steel bands cause the molded polyurethane layer to have a non-uniform thickness, as explained in more detail below with reference to FIG. 1.
FIG. 1 is a simplified top view of a conventional mold for manufacturing a stainless steel reinforced belt pad for a linear CMP system. As shown therein, mold 100 includes outer mold 102 and inner mold 104. Outer mold 102 is in the form of a hollow cylinder and inner mold 104 is in the form a solid cylinder. To begin the molding process, stainless steel band 110 is first placed around inner mold 104. Although the specified diameter and tolerances of stainless steel band 110 are selected so that the stainless steel band will fit snugly around inner mold 104, significant variations in the size and roundness of the manufactured stainless steel band often occur. When stainless steel band 110 does not fit snugly around inner mold 104, space 108 may be formed between the stainless steel band and the inner mold. It is to be understood that the size of space 108 shown in FIG. 1, which is not drawn to scale, is exaggerated for purposes of illustration. Thereafter, when the pad material, e.g., polyurethane, is injected into gap 106 between outer mold 102 and stainless steel band 110, the resulting layer of polyurethane will not have a uniform thickness because the distance between the stainless steel band and the outer mold is not uniform.
One way to ensure that the layer of polyurethane has a uniform thickness would be to machine the layer to a uniform thickness after the molding process. This solution is undesirable for at least two reasons. First, the additional machining step increases the complexity and cost of the molding process. Second, the layer of polyurethane cannot be machined beyond a required minimum thickness. Therefore, if the thin spots in the layer of polyurethane are too exaggerated, then machining back to uniformity is not a viable option.
In view of the foregoing, there is a need for a process for molding stainless steel reinforced belt pads for linear CMP operations that ensures that the pad has a uniform thickness.